Portland clinker, the base of cement, abundant and cheap, is possibly the most manufactured processed material in the world. In the laboratory we have transformed it into garnet, in a similar way to how artificial diamonds are obtained from graphite, applying pressure and temperature.
The process is yet another breakthrough in pioneering technologies that are revolutionizing materials as we know them. We are not alchemists or magicians: we transform cement into garnet with the only spell of the advancement of science.
How is it possible? Slight mineralogical variations in the initial compositions can produce drastic changes, both in the final mineralogical composition and in the morphology of the material. Thus, we have managed to obtain artificial garnets in the laboratory by applying pressures as low as 50 megapascals. In nature much higher pressures are required for them to occur.
From limestone and clay
Portland clinker is the raw material from which cement is made. It is a ceramic material that is obtained after applying heat to a mixture of limestone and clay, very abundant and cheap materials in nature.
One time clinkerized A mixture of ceramic components is achieved, among which there are mainly silicates (dicalcium and tricalcium silicate), tricalcium aluminate and tetracalcium ferrite-aluminate. Once transformed, it must be mixed with plaster and some other additive to obtain hydraulic cement, the main material used in construction. The clinker manufacturing process is massive: more than 4 billion tons per year worldwide. It is possibly the most manufactured processed material on the face of the earth.
Clinker, abundant and cheap, has a price close to 60 euros per ton.
Garnet: the jewel in the color of pomegranate
Garnet is a semi-precious stone, and its chemistry is very complex. It owes its name to the fact that it has a color similar to that of the grains of the fruit known as pomegranate. It is used in jewelry and also as an abrasive. Garnet is harder than quartz. It is not as scarce as other precious or semi-precious stones (more than 400,000 tons are produced per year), but it has a market value six times that of clinker.
The composition of garnet is based on silicates, but it is much more complex than that of clinker and has a different crystalline structure.
Turning clinker into garnet has required almost 30 years of research work and something really significant needs to be highlighted: it was not what we were looking for.
A starting point
In 1995 we published an article in the Bulletin of the Ceramic Society that studied the feasibility of using this cheap raw material, clinker, in the manufacture of structural ceramics using a conventional method: uniaxial pressing and sintering. That work was the starting point that demonstrated that in addition to being resistant, the ceramic thus obtained is quite reliable and reproducible.
We already knew then that the material obtained had mechanical properties that could be competitive, and the idea gave rise to some interesting work, but it did not translate into any industrial application.
Almost thirty years later, a collaborative work (among UC3M, UPM, CINN-CSIC, Universidad Europea and Inop-Poznan) led by two research groups, the IMDEA Materials Institute and the Madrid Materials Science Institute (ICMM-CSIC) , has consolidated the material using an advanced sintering technique called Spark Plasma Sintering.
pressure and temperature
The Spark Plasma Sintering It is a technology that uses, simultaneously, pressure and temperature. The temperature increase is achieved thanks to the passage of a pulsed current through the clinker powder.
Electric Field Assisted Sintering (SPS) equipment is commercially available and is no longer limited to laboratory research work. Products such as thermoelectric semiconductors, gradient function materials, biomedical implants, turbine blades or rocket nozzles can be produced industrially using this equipment.
If using more conventional techniques with ceramics the results had been promising, it was to be expected that using SPS they could be improved. So it was. But, in addition, we have obtained something unexpected: a material with the same properties as a semiprecious stone, garnet. Like many other times in science, serendipity has once again surprised us.
The joint work has been published in Ceramics International and shows that this material, consolidated with not very high pressure and temperatures, can be partially transformed into garnet.
The recipe to obtain garnet
Only the increase of a few hundred degrees of temperature applied simultaneously to a few dozen MPa of pressure has worked the miracle of the transformation of an unpretentious material into garnet, a semi-precious material that humans have used since the Bronze Age.
At sintering temperatures, liquid cavities are created within a solid network which, upon cooling, become high-pressure zones. In this way, in some localized areas the pressure is amplified considerably and the miracle.
In a similar way to how artificial diamonds are obtained from graphite (for this, high temperatures and many GPa of pressure must be applied simultaneously), but using much more industrially accessible pressures and temperatures, it is possible to transform Portland clinker into garnet.
Materials science is making its way by studying routes already explored and taking advantage of the technological leaps available to add value to materials that are abundant and cheap as raw materials. From cement to garnet. It seems impossible, right?